1. Field of the Invention
The present invention relates to a channel estimating apparatus and method in a Broadband Wireless Access (BWA) communication system. More particularly, the present invention relates to an apparatus and a method for more accurately estimating a channel of adaptive Infinite Impulse Response (IIR) type by selecting a necessary filter factor in a receiver of an Orthogonal Frequency Division Multiplexing (OFDM) communication system.
2. Description of the Related Art
According to an OFDM scheme, a transmitter Quadrature Phase Shift Key (QPSK)-modulates or Quadrature Amplitude Modulation (QAM)-modulates frequency-domain digital information and converts the modulated information to a time-domain signal using an Inverse Fast Fourier Transform (IFFT) operation. In doing so, the transmitter typically sends a pilot subcarrier between a preamble symbol and a data subcarrier so that a receiver can easily estimate the channel. The receiver estimates the channel for the data subcarrier using the preamble symbol and the pilot subcarrier and uses the estimate value for the demodulation.
When an OFDM scheme is adopted in a mobile communication, a reception performance of the receiver mostly relies on how accurately the receiver estimates the varying channel while traveling around.
A frame structure in an Institute of Electrical and Electronics Engineers (IEEE) 802.16 Orthogonal Frequency Division Multiple Access (OFDMA) scheme is illustrated by referring to FIG. 1. FIG. 1 depicts a pattern of preamble and pilot subcarriers of a downlink in a start interval of a frame in a conventional OFDMA system.
The downlink of the OFDMA system carries frames each including preamble and pilot subcarriers per frame to the receiver. The receiver estimates a channel of the neighbor data subcarrier using the received frame as shown in FIG. 2.
FIG. 2 is a flowchart of the channel estimation in a conventional channel estimating apparatus in the OFDMA system. Upon receiving the frame, the conventional channel estimating apparatus estimates channel estimate values for every subcarrier of the symbol index 0 by interpolating preamble subcarriers along a frequency axis in step 200 and estimates subcarrier estimate values of the neighbor symbol using a time-axis interpolation in step 202.
In step 202, the subcarrier channel estimate value of the symbol index 1 corresponding to the pilot subcarrier position k of the symbol index 2 is calculated by interpolating the subcarrier channel estimate value of the neighbor symbol along the time axis based on Equation (1).
                                          H            1                    ⁡                      (            k            )                          =                                            1              2                        ⁢                                          H                0                            ⁡                              (                k                )                                              +                                    1              2                        ⁢                                          H                2                            ⁡                              (                k                )                                                                        (        1        )            
In Equation (1), Hi(j) denotes a j-th subcarrier estimate value of an i-th symbol index.
The channel values of the subcarriers of the symbol index 2 can be acquired in the same manner. Accordingly, Equation (1) can be generalized as below; that is, the subcarrier estimate value of the symbol index n corresponding to the pilot subcarrier position k of the symbol index n+1 can be acquired by interpolating the subcarrier estimate value of the neighbor symbol along the time axis based on Equation (2).
                                          H            n                    ⁡                      (            k            )                          =                                            1              2                        ⁢                                          H                                  n                  -                  1                                            ⁡                              (                k                )                                              +                                    1              2                        ⁢                                          H                                  n                  +                  1                                            ⁡                              (                k                )                                                                        (        2        )            
In Equation (2), Hi(j) denotes a j-th subcarrier estimate value of an i-th symbol index.
Next, the conventional channel estimating apparatus estimates the channel for every subcarrier of the symbol index by applying the frequency-axis interpolation using the subcarrier estimate values acquired using the pilot symbols and the time-axis interpolation in step 204, and determines whether the channel is estimated up to the end of the frame in step 206. When the channel estimation is not completed in step 206, the conventional channel estimating apparatus goes back to step 202. Otherwise, the conventional channel estimating apparatus finishes the process of FIG. 2.
However, disadvantageously, the channel estimating method of FIG. 2 performs the one-dimensional time/frequency-axis interpolations without considering the channel condition and makes use of the previous symbol estimate value such as preamble for the next symbol channel estimation even when the temporal change of the channel is not so great (e.g., low-speed environment).
Accordingly, there is a need for an improved apparatus and the method for estimating a channel by selecting a necessary filter factor when the receiver of the BWA communication system estimates a channel of an adaptive IIR type, which can provide a far more accurate channel estimate value by promptly adapting to the channel change.